I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for coherent demodulation in a wireless communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Pat. No. 5,414,796, entitled xe2x80x9cVARIABLE RATE VOCODERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when the speech encoder is providing speech data at a maximum rate. When the variable rate speech encoder is providing speech data at a less than maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate, is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Pat. No. 5,659,569, entitled xe2x80x9cDATA BURST RANDOMIZERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the wireless communication device at a base station. In response to the monitored power level, the base station transmits power control bits to the wireless communication device at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
Within a coherent demodulator, is a channel estimate generator (not shown), which estimates the channel characteristics based on a transmitted signal with values known to both the transmitter and the receiver, referred to herein as the pilot signal. The pilot signal is demodulated and the phase ambiguities in the received signal are resolved by taking the dot product of the received signal and the pilot signal channel estimate. A description of a circuit for employing a pilot signal to resolve phase ambiguities is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUITxe2x80x9d, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a wireless communication device to a central base station is to allow the wireless communication device to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the wireless communication device to transmit its information using a small set of orthogonal channels. Such a method is described in detail in co-pending U.S. patent application Ser. No. 08/886,604, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d (hereafter the ""604 application), assigned to the assignee of the present invention and incorporated by reference herein.
In the ""604 application, a system is disclosed in which a pilot signal is transmitted on the reverse link (the link from the wireless communication device to the base station) to enable coherent demodulation of the reverse link signal at the base station. Using the pilot signal data, coherent processing can be performed at the base station by determining and removing the phase offset of the reverse link signal. Also, the pilot data can be used to optimally weigh multipath signals received with different time delays before being combined in a RAKE receiver. Once the phase offset is removed, and the multipath signals are properly weighted, the multipath signals can be combined to decrease the power at which the reverse link signal must be received for proper processing. This decrease in the required receive power allows greater transmission rates to be processed successfully, or conversely, the interference between a set of reverse link signals to be decreased.
While some additional transmit power is necessary for the transmission of the pilot signal, in the context of higher transmission rates the ratio of pilot signal power to the total reverse link signal power is substantially lower than that associated with lower data rate digital voice data transmission cellular systems. Thus, within a high data rate CDMA system, the Eb/N0 gains achieved by the use of a coherent reverse link outweigh the additional power necessary to transmit pilot data from each wireless communication device.
However, when the data rate is relatively low, a continuously-transmitted pilot signal on the reverse link contains more energy relative to the data signal. At these low rates, the benefits of coherent demodulation and reduced interference provided by a continuously-transmitted reverse link pilot signal may be outweighed by the decrease in talk time and system capacity in some applications.
The present invention is a novel and improved method and system for performing the pilot frequency tracking operation for coherent demodulation in a system employing a gated pilot signal. In particular, the present invention describes a method and apparatus in which two frequency tracking loops operate in parallel. The first frequency tracking loop performs its tracking operation based on the hypothesis that the received pilot is continuous throughout the frame""s duration. The second frequency tracking loop performs the tracking operation based on the hypothesis that the received pilot is discontinuous and is only present for a portion of the frame""s duration.